Hottest weather in 600 years recorded

The 1990s are giving us some of the hottest weather since 1400, according to research which ranks 1997, 1995 and 1990 as the warmest on record in the northern hemisphere since the Middle Ages.

Climatologists at the University of Massachusetts at Amherst used data from tree rings, polar ice cores, coral reefs and historical records to estimate yearly temperatures to a fraction of a degree back to AD1400. Dr Michael Mann and colleagues publish their findings in the current issue of Nature.

Their work indicated major climate change in 1791 when a strong El Nino is thought to have occurred, a finding which matches existing historical documents. They also found 1816 writ large in the environmental record; the "year without a summer" which followed the eruption of the Mount Tambora volcano in Indonesia.

The researchers point to human activity, particularly the release of greenhouse gases, as a cause of rising average temperatures. "The anomalous warmth of several recent years appears likely to be related to human influences on climate," Dr Mann said.

This change has been reflected in Irish records, according to Met Eireann. "1997 is the one that stands out. It was a very warm year here," said Mr Denis Fitzgerald, head of climatology at Met Eireann.

The warmest two years were 1945 and 1949, he said, but many monthly and seasonal records had been set during the 1980s and 1990s. The "hottest summer" record was set and reset seven times during the 1980s and several times during the 1990s.

Abrupt and dramatic climate change is nothing new for the Earth, however, according to scientists at the Massachusetts Institute of Technology. They studied the geological record looking at climate trends back to the dawn of mankind, 1.5 million years ago.

They detected huge temperature variations, up to 10C, occurring over a few decades, according to research published earlier this month. This would be like Ireland suddenly taking on Spanish resort weather within a 25-year period.

The MIT group, led by Dr Maureen E. Raymo, suggested that after more than 8,000 years of warmth, the planet is due for an Ice Age-style cooling phase. "We're living at the end of an unusually stable period, but all bets are off because of global warming from the build-up of carbon dioxide or other greenhouse gases in the atmosphere," she said.